1. Field of the Invention
The present invention relates to a multibeam scanning system for use in an apparatus for recording an image, such as a laser plotter and a color image scanner. In particular, the present invention relates to a multibeam scanning system with a beam interval adjuster for changing an interval of beam spots on a recording surface while maintaining a spot diameter.
2. Description of the Prior Art
An apparatus for recording an image, such as an image scanner and a printer, often includes a multibeam scanning system for focusing a plurality of optical beams on a photosensitive material to expose the same. The use of a multibeam scanning system reduces exposure time. When an image to be reproduced is a halftone image, each halftone dot is formed with a prescribed number of scanning lines.
An image scanner used in a printing process must be able to change an interval of halftone dots formed on a reproduced image. Even when the interval of halftone dots is changed, the number of scanning lines needed to form one halftone dot is not usually changed. Accordingly, the interval of beam spots on photosensitive material should be changed according to the interval of halftone dots.
An apparatus for changing the interval between beam spots is described in Japanese Patent Laying Open Gazette No. 60-169820. Since this apparatus changes the interval between beam spots with a zoom lens, the spot diameter increases as the interval between beam spots increases.
However, when the spot diameter increases, the distribution of the intensity of the beam spots becomes flatter. For example, a spot diameter of 20 .mu.m becomes 60 .mu.m when the interval between beam spots is multiplied by three with a zoom lens. If halftone dots are formed with these large optical beams, the boundaries of the halftone dots are blurred. This problem is especially important in connection with a flat bed type image scanner, which scans a long scanning line in every main scanning operation.
Therefore, it is desirable to change the beam spot interval while maintaining the beam spot diameter, especially in a flat bed type image scanner.